The magnitude distributions of galaxies identified with faint radio
sources are often used to estimate redshift distributions. These
estimates can be refined if broadband
colors and optical morphologies are available, since the absolute
magnitudes of the giant elliptical galaxies associated with the most
luminous radio sources are
somewhat brighter than those of elliptical galaxies identified with less
luminous sources
(Auriemma et al. 1977)
and the "blue" population of galaxies investigated
by Kron et al. (1985).
Windhorst et al. (1984a)
have shown that the fraction of
radio sources identified with galaxies brighter than
J 23.7
is about 50%, nearly
independent of 1.4-GHz flux density in the range of 1 to 100 mJy. Thus,
the median redshift in this flux-density range is roughly equal to the
redshift of a giant elliptical galaxy at their plate limit,
z 0.8. Unless
the amount of evolution changes
discontinuously at redshifts just beyond
z 0.8, most of the
remaining objects fainter than
J 23.7 must
have redshifts only
slightly larger. In fact, the identification rate approaches 100% at
V 26
(Windhorst 1986),
and few galaxies in this
flux-density range should have redshifts greater than 1.9.